That's partly because a slew of temporary tax cuts enacted during the Great Recession have started to expire. And it's partly because an improving economy means people's incomes should slowly start to increase, adding to their income tax bill.

By 2024, the tax policy think tank projects that only about one-third of households won't be paying any federal income taxes. The improvements might take a while because the economy has been adding new jobs at a painfully slow pace, and many workers aren't yet seeing much bigger paychecks.

Romney made "the 47 percent" famous during his 2012 Republican presidential campaign, after he was secretly recorded at a closed-door fundraiser saying that 47 percent of the population is dependent on government, believes the government has a responsibility to care for them "and will vote for this president no matter what."